Subtract.
Explanation: There are many ways to solve this problem. Let's see two ways we can solve it. Place Value We can think in terms of hundredths: $\begin{aligned} &\phantom{=}79.18 - 7\\\\ &=79.18-7.00\\\\ &=7{,}918\text{ hundredths} -700\text{ hundredths}\\\\ &=7{,}218\text{ hundredths}\\\\ &=72.18 \end{aligned}$ Place value table Let's line up the numbers by their decimal places. Tens Ones $.$ Tenths Hundredths ${7}$ ${9}$ $.$ ${1}$ $8$ $7$ $.$ ${0}$ $0$ Now, let's subtract each place value column. Tens Ones $.$ Tenths Hundredths ${7}$ ${9}$ $.$ ${1}$ ${8}$ $-$ $7$ $.$ ${0}$ $0$ $\underline{~~~~}$ $\underline{~~~~}$ $\underline{~~~~}$ $\underline{~~~~}$ $7$ $2$ $.$ $1$ $8$ $72.18 = 79.18 - 7$